Size: HTML5 Ads are measured as file size in whole including the back up images, click tags/codes and other including files whereas Flash ads weight limit is based on the creative size only. Thus, HTML5 ads are heavy in size and can be around 100Kb +.
Optimization: Flash automatically provides optimization for codes/assets and compiles everything into a single file which is already compact with minimal number of files. For HTML5, no optimization tools are available yet. Thus, special attention shall be given on how HTML5 ads are packaged and delivered as it helps in quick ad load time.
Parent company: Flash is not an open standard, it is controlled by ADOBE systems whereas HTML5 is largely controlled by a committee managed by – MOZILLA, OPERA SOFTWARE and APPLE.
Cost: Constructing flash websites using adobe tools is expensive than HTML5. Making a website using HTML5 may require a lot of LOCs but is not expensive as flash websites
Convenience: HTML5 can render multimedia content easily without the necessity of installing a plugin or player application unlike FLASH. Though HTML5 does have a downside to this, namely the fact that older browsers don’t render it correctly or don’t render it at all.
Picture Clarity: Flash has the ability to specify measures in subpixel which results in crisp and more pleasant appearance whereas HTML5 can lead to inconsistency and unreliability in display.
Mobile: Most important feature of HTML5 is its capability to run flexibly on Mobile because of which it’s getting preference above FLASH as Flash is bound to PC only. The same ad unit can be used across devices.
Development: Flash has a larger resource pool and even larger community whereas HTML5 is fairly new technology so its compatibilities are quite limited.
Usability: Amount of processing power required to run HTML5 is considerably lower than Flash given the fact that more number of people are using mobile devices. Designers are also aiming to create different versions of websites for PC and mobile (Stripped down version).
Semantic Elements in HTML5 is the use of HTML markup to reinforce the semantic/meaning of the information in webpages and web applications rather than merely to define its presentation or look which can’t be done via non-semantic elements i.e. <div>, <span>, etc. This helps in processing faster as browser gets to know the purpose by the element whereas no such functionality in flash.